Is fear of having drugs planted in my bag enroute to Malaysia justified?

Firstly, yes currently, drug trafficking carries a mandatory death sentence there (although only if you're carrying more than a certain amount, out of interest).

Worth considering, however:

• Millions of people fly there every year. Very few are caught as drug smugglers, and I would presume even fewer have drugs put in their bag, and even fewer still would get 'caught' like that.
• You can always lock your bag, wrap it in tape, or otherwise make it harder to break into. If you were looking for a bag to repurpose for drugs, you're presumably more likely to look for one easy to open.
• I've been to Malaysia with drugs - prescription drugs. I declared them openly and they didn't bat an eyelid. Didn't even ask to see them. So don't get worried about prescription drugs vs illegal narcotics, like many seem to (although that doesn't seem to be your concern, I'm leaving it for future readers)
• If you're very concerned, you could search your own bag on arrival before you go through security. If by some horribly small chance you did find illegal drugs planted there, we have a question on how to handle that.
• You could consider travelling only with carry-on. I did that in April this year for a holiday to Thailand, and it was frankly liberating, not having to lug a bag around. Obviously it depends on your needs, but it meant I could keep my eyes on my bag at all times during the flight (I have it by my legs).

Finally, and I'm happy to be proven wrong, but I'd be willing to bet large sums that statistically, you're more likely to die in a car crash than have drugs planted in your bag entering Malaysia. If you're willing to get into a car tomorrow, you don't need to worry about this. Sometimes rationalising it helps. However, sometimes anxiety isn't rational, and I get that. But it's honestly not worth worrying about, look forward to your amazing holiday instead!

Just to add one more point to Mark Mayo's excellent Answer, this would be concern everywhere, not just Malaysia.

However, planting any contraband in passenger baggage is very uncommon because it is comparatively risky for the smugglers. There is a much higher chance of either losing control, as opposed to airline or supporting assets such as catering equipment, or having it detected by either the passenger or entry inspection.